Engine oil composition

ABSTRACT

The present invention is directed to provide an engine oil composition which fully exploits the performance of molybdenum dithiocarbamate (MoDTC), restricts degradation of MoDTC itself, and has a high MoDTC residual property even at the time of degradation, hence providing low friction and low wear for a long time, and which results in savings in fuel consumption. It is also directed to an engine oil composition having a high coefficient of friction and an extreme-pressure property under fluid lubricating conditions arising from extreme-pressures, in order to solve various problems encountered in friction. The engine oil compositions according to the present invention comprise a specific MoDTC, a specific zinc dithiophosphate and a base oil for engine oil as the essential components, and specific polyglycerin half esters may be further added.

INDUSTRIAL FIELD OF APPLICATION

This invention relates to an engine oil composition, more specifically,to an engine oil composition which is produced by blending molybdenumdithiocarbamate (hereinafter referred to as "MoDTC") and zincdithiophosphate containing a primary alkyl group having 8 to 14 carbonatoms (hereinafter referred to as "ZnDTP") to a base oil for an engineoil, which has high residual MoDTC even when the oil degrades, henceproviding low friction and low wear over a long period of time, leadingto lower fuel consumption. The invention also relates to an engine oilcomposition which is produced by blending MoDTC, ZnDTP and polyglycerinhalf ester to a base oil for an engine oil, that is stable under fluidlubricating conditions from extreme pressure conditions and which has anexcellent coefficient of friction.

PRIOR ART

Improvements in engine oils have been attempted in the past because ofthe promotion of energy conservation and technological progressesrelated to higher performance and higher output from automobiles, butthe environment for engine oils has become more severe due to the riseof oil temperatures resulting from higher speeds and higher outputs ofengines, deterioration of friction conditions, the limitations on oilcapacity due to reductions in weight, the requirements formaintenance-free operation resulting from long drain, etc.

Engine oils play an important role in valve actuating systems, bearings,etc., in addition to their function as a lubricant between pistons andliners. Lubricating conditions differ depending on portions of theengine, and the performance required for engine oils has becomediversified. In the piston portion, for example, a fluid lubricatingcondition is predominant. In this case, a lower viscosity engine oilplays the greatest role in reducing friction loss. When the viscosity ofthe engine oil is reduced, however, sealability deteriorates and wearincreases. In the valve actuating system, on the other hand, lubricationis mainly mixed lubricating and boundary lubricating conditions.Accordingly, because reductions in engine oil viscosity has a negativeeffect on wear, additives having high extreme-pressure performance andhigh wear resistance become necessary.

In addition, the regulations on fuel consumption of automobiles and therestrictions on exhaust gases have become more severe due toenvironmental problems such as the greenhouse effect, emissions ofnitrogen oxides (NO_(x)), etc. For these reasons, further improvementsin mechanical efficiency such as from reductions in engine oil viscosityand excellent friction regulating additives are being sought.

As the viscosity of engine oils has been reduced, MoDTC and ZnDTP havebeen employed so as to reduce frictional loss, to prevent wear and toimpart extreme-pressure properties, as additives for the base oil forengine oil. However, when these additives are merely mixed, theresulting engine oil compositions cannot substantially solve suchproblems as exhaust gas emissions, wear associated with the restrictionson fuel consumption drops in mechanical efficiency resulting fromseizure and frictional loss, etc.

MoDTC undergoes deterioration as the oil deteriorates and eventuallyloses its friction reduction effect. Therefore, how to maintain theperformance of MoDTC, particularly in engine oils, has been a criticalproblem that is yet to be solved. From the aspect of reducing engine oilviscosity or the fuel consumption by friction regulating additives,however, the use of MoDTC is essentially necessary at the presentmoment. In order to solve such problems as wear, drops in mechanicalefficiency due to seizure and frictional loss, etc., therefore, it isnecessary to fully exploit the performance of MoDTC, and from the aspectof long drain, too, an oil which maintains the performance of MoDTC evenwhen the oil degrades and which exhibits a friction reduction effect fora long time must be developed.

In connection with ZnDTP, J. A. Spearot, F. Caracciolo et al report inSAE Paper 790941 (1979) that phosphorus (P) in engine oils lowers thefunctions of catalysts and O₂ sensors and deteriorates the purificationratios of CO, HC and NO_(x) in exhaust gas. At present, attempts havebeen vigorously made to reduce the P content on the basis of theobservation described above, but when wear resistance is considered inconjunction with the lower viscosity of engine oil, the addition ofZnDTP as a wear-proofing agent becomes inevitable. Even so, oils havinga normal P content of more than 1,200 ppm are not presently being usedas engine oils.

Under the circumstances described above, Japanese Patent Laid-OpenNo.63-178197 proposes a lubricating oil composition for a powertransmission apparatus having a traction drive mechanism whichcomposition is obtained by blending MoDTC and ZnDTP having a primaryalkyl group to a base oil consisting of saturated hydrocarbon compoundshaving a condensed ring and/or an uncondensed ring as its principalcomponent. In the composition of this patent application, however, theoil is a lubricating oil for the power transmission apparatus having thetraction drive mechanism, though the composition uses MoDTC and ZnDTP.Since the application of this lubricating oil is different from that ofan engine oil, its basic oil is specific, and performance as an engineoil cannot be expected.

Japanese Patent Publication No.3-23595 proposes a lubricating oilcomposition prepared by blending 0.2 to 5 percent by weight of MoDTC,0.1 to 7 percent by weight of ZnDTP (at least 50% of which consists ofZnDTP having a secondary alkyl group), 0.1 to 20 percent by weight ofcalcium alkylbenzenesulfonate and 1 to 15 percent by weight ofalkenylsuccinimido to 98.6 to 53 percent by weight of a mineral oiland/or synthetic oil having a kinematic viscosity ranging from 3 to 20cSt at 100° C.

Japanese Patent Laid-Open No.62-275198 proposes a composition preparedby adding 3 to 10 percent by weight in total, of an organomolybdenumcompound, organozinc compound and aryl phosphate, each being soluble ina base oil for lubricant, to said base oil, and a lubricant prepared byblending the composition in a weight ratio of 0.5 to 1.5(organomolybdenum compound):0.5 to 1.5 (organozinc compound):0.5 to 1.5(aryl phosphate).

Japanese Patent Laid-Open No.5-279688 teaches that frictioncharacteristics can be improved without reducing wear resistance andother characteristics by blending an organomolybdenum compound,aliphatic ester, metal detergent (calcium or magnesium sulfonate,calcium or magnesium phenate), ashless detergent-dispersant(benzylamine, alkenylsucciniimide, boron derivative ofalkylsucciniimide) and wear-proofing agent (zinc dithiophosphate, zincdithiocarbamate).

Japanese Patent Laid-Open No.5-311186 teaches that the coefficient offriction of a lubricating oil can be drastically lowered by blendingsulfurized oxymolybdenum dithiocarbamate and/or sulfurized oxymolybdenumorganophosphorodithioate; an aliphatic ester and/or an organoamidecompound in specific amount ratios with a combination system of a metaldithiocarbamate having not greater than 4 carbon atoms with anoil-soluble amine compound.

Problems the Invention Aims to Solve

However, although the composition of Japanese Patent PublicationNo.3-23595 has high initial performance, its performance drops withdegradation of the oil. Thus, this prior art cannot solve the problemsdescribed above, and improvements are left yet to be made.

Japanese Patent Laid-Open No.62-275198 describes that MoDTC, ZnDTP andaryl phoaphate preferably exist specifically in a weight ratio of about1:1:1, and that the total weight in the final lubricant (that is, thetotal of the three components) is particularly from 3.9 to 9.9%, moreparticularly 5.9 to 7.9% such as about 6.9%. In the compositiondescribed above, however, the amounts of addition of both MoDTC andZnDTP are so great that the problems of friction resistance and wearresistance are left yet to be improved. As also described already, thereduction of the P content has been made vigorously in engine oils, andoils having a F content of higher than 1,200 ppm are not generallyemployed. For this reason, too, the composition described above cannotbe used for engine oils.

Moreover, none of these patent applications study the behaviour of MoDTCwith degradation of the oil, and it is doubtful whether the performanceof MoDTC can be maintained at the time of oil degradation. Further,performance of residual MoDTC has become more important at the time ofoil degradation with increases in the term of long drain.

Further, the compositions described in the above patent applications donot completely solve the various problems with engine oils describedabove. In other words, the use of MoDTC is essential at the present timefrom the aspects of lower viscosity engine oils or saving energy coststhrough friction regulating additives. Also, it is very important tofind a composition which fully exploits the performance of MoDTC inorder to solve the various problems due to drops in mechanicalefficiency from friction, seizure and friction loss.

It is therefore an object of the present invention to provide an engineoil composition which fully exploits the performance of MoDTC, restrictsthe degradation of MoDTC itself, has a high residual MoDTC property evenat the time of oil degradation, provides low friction and low wear for along term and results in reduced fuel consumption, in order to solve thevarious problems with friction.

It is another object of the present invention to provide an engine oilcomposition which fully exploits the performance of MoDTC and has anexcellent coefficient of friction and extreme-pressure properties underfluid lubricating conditions from extreme-pressure conditions.

Means of Solving the Problems

To accomplish the objects described above, the present inventors haveconducted studies and have found out that the performance of MoDTC canbe extended and that low friction as well as low wear can be achievedover long periods by combining MoDTC with ZnDTP having a primary alkylgroup with 8 to 14 carbon atoms. Thus, a first embodiment of the presentinvention has been completed.

In other words, an engine oil composition according to the firstembodiment of the present invention comprises, as the essentialcomponents:

(A) at least one kind of molybdenum dithiocarbamate (MoDTC) representedby the following general formula (1): ##STR1## wherein each of R¹, R²,R³ and R⁴ may be the same or different and each represent an alkyl grouphaving 8 to 16 carbon atoms, X represents a sulfur atom or oxygen atom,and a ratio of the sulfur atoms to the oxygen atoms is from 1/3 to 3/1;

(B) at least one kind of neutral or basic zinc dithiophosphate (ZnDTP)represented by the following general formula (2) wherein the proportionof zinc dithiophosphate whose R, which may be the same or different andrepresents a primary alkyl group having 8 to 14 carbon atoms, is atleast 50 percent by weight in all the zinc dithiophosphates:

    Zn (RO).sub.2 PS.sub.2 !.sub.2 •aZnO                 (2)

wherein a is 0 or 1/3 and R may be the same or different and representsan alkyl group having 3 to 14 carbon atoms: and

(C) a base oil for engine oil;

wherein the proportion of the Component (A) is 0.03 to 1 parts by weightbased on 100 parts by weight of base oil for the engine oil, and theproportion of the Component (B) is 0.01 to 2 parts by weight.

In the engine oil composition according to the present invention, it isparticularly preferred that all R groups in the general formula (2) be2-ethylhexyl groups.

Also, to accomplish another of the objects described above, the presentinventors have conducted intensive studies and have found out thatsurprising lubricating performance can be obtained by combining MoDTC,ZnDTP and a certain kind of half ester of a particular fatty acid (inthe present specification, a polyhydric alcohol in which part of thehydroxyl groups in said alcohol are esterified will be called a "halfester"). Thus, a second embodiment of the present invention has beencompleted.

In other words, the engine oil composition according to the secondembodiment of the present invention is prepared by blending 0.1 to 5parts by weight of at least one kind of polyglycerin half estersrepresented by the following general formula (3) to 100 parts by weightof a base oil for engine oil: ##STR2## wherein n is an integer of 1≦n≦9.R⁵ to R⁸ each represent hydrogen atoms or an acyl groups having 8 to 20carbon atoms with the provision that all R⁵ to R⁸ are neversimultaneously either all hydrogen atoms nor all acyl groups, andindividual R⁸ 's may be the same or different when n is 2 or more.

When importance is attached to the extreme-pressure property of theengine oil composition in the second embodiment of the presentinvention, it is preferred that the polyglycerin half esters are atleast one kind in which the number (Y) of the acyl groups in the generalformula (3) is within the range of 1≦Y≦(n+5)/2 polyglycerin half estersof this kind will hereinafter be called "polyglycerin half esters (I)"!.

In the second embodiment of the present invention, it is furtherpreferred that the polyglycerin half ester is at least one kind in whichthe proportion of lauryl groups and/or oleyl groups to all the acylgroups in the general formula (3) is at least 25% polyglycerin halfesters of this kind will hereinafter be called "polyglycerin half esters(II)"!.

Further, in the second embodiment of the present invention, it is mostpreferred that the polyglycerin half esters are at least one kind inwhich the acyl groups in the general formula (3) are all oleyl groupsand/or lauryl groups polyglycerin half esters of this kind willhereinafter be called "polyglycerin half esters (III)!.

Embodiments

In MoDTC represented by the general formula (1) as the Component (A)used in the present invention, the hydrocarbyl groups represented by R¹,R², R³ and R⁴ may contain saturated or unsaturated bonds and may be astraight chain type, a branched chain type or ring-like, or combinationsthereof. Though they may contain 8 to 16 carbon atoms in some cases fromthe aspect of lubricating properties, they preferably contain 8 to 13carbon atoms with 8 carbon atoms being particularly suitable.

Such hydrocarbyl groups are aliphatic groups, aromatic groups andaromatic-aliphatic groups. More concretely, they are alkyl groups suchas an octyl group, 2-ethylhexyl group, nonyl group, decyl group, dodecylgroup, lauryl group, tridecyl group, isotridecyl group, tetradecylgroup, pentadecyl group, hexadecyl group, and so forth. Preferred amongthem are the 2-ethylhexyl group, octyl group, tridecyl group andisodecyl group, and further preferred are those in which R¹, R², R³ andR⁴ are a 2-ethylhexyl group.

Further, in MoDTC represented by the general formula (1), none of theX's are simultaneously O or S. In other words, the ratio S/O is withinthe range of 1/3 to 3/1. If all of the X's are oxygen, the lubricatingproperty becomes inferior, and if all of the X's are sulfur, corrosionis more likely to develop.

(A) MoDTC represented by the general formula (1) is used in an amount of0.03 to 1 part by weight, preferably 0.1 to 0.6 part by weight based on100 parts by weight of the base oil for engine oil. If the amount isless than 0.03 parts by weight, the reduction of the coefficient offriction is not sufficient and if it exceeds 1 part by weight, a furthereffect of reducing the coefficient of friction cannot be obtained, andconversely adverse influences such as the occurrence of sludge tend tooccur.

Such (A) MoDTC can be produced by the methods described, for example, inJapanese Patent Publication Nos.53-31646, 55-40593, 56-12638, 57-24797,58-50233 and 62-81396.

In ZnDTP as the Component (B) represented by the general formula (2)used in the present invention, a is zero or 1/3. When a=zero, thecomponent is generally called a "neutral salt" and when a=1/3, it isgenerally called a "basic salt". The (B) ZnDTP used in the presentinvention may be a neutral salt, a basic salt or combinations thereof.

In (B) ZnDTP represented by the general formula (2) used in the presentinvention, the hydrocarbyl group represented by R may contain saturatedor unsaturated bonds having 3 to 14 carbon atoms, and may be a straightchain type, a branched chain type, a ring-type or combinations thereof.Further, the hydrocarbyl groups may be the same or different, but theproportion of ZnDTP in which all of the R groups are primary alkylgroups having 8 to 14 carbon atoms (they may be the same or different)in all the ZnDTPs must be at least 50 percent by weight.

Such hydrocarbyl groups include aliphatic types, aromatic types andaromatic-aliphatic types. Concrete examples include alkyl groups such asan octyl group, 2-ethylhexyl group, nonyl group, decyl group, laurylgroup, tridecyl group, tetradecyl group, etc; cycroalkyl groups such asa cyclohexanethyl group, etc; and aryl groups such as analkyl-substituted phenyl group (for example, phenylmethyl group,phenylethyl group and xylyl group). The hydrocarbyl groups arepreferably a 2-ethylhexyl group, octyl group, nonyl group and tridecylgroup and most preferably, all of the R groups are 2-ethylhexyl andoctyl groups.

These (B) ZnDTPs may be used either individually or in combinations oftwo or more in mixture. Though they function as an extreme-pressure,agent, anti-oxidant, corrosion inhibitor, etc., the effect of thepresent invention cannot be obtained unless at least 50 percent byweight of ZnDTP having the primary alkyl group is added. The greater thecontent of ZnDTP whose primary alkyl groups are all 2-ethylhexyl groupsor octyl groups, the higher the MoDTC residual effect becomes.

The (B) ZnDTP represented by the general formula (2) is used in theamount of 0.01 to 2 parts by weight based on 100 parts by weight of thebase oil for engine oil. If the amount is less than 0.01 part by weight,the effect of improving the MoDTC (A) residual property is notsufficient and if it exceeds 2 parts by weight, the coefficient offriction at the time of degradation of the base oil or the engine oildeteriorates. If the amount added is great, the catalyst of an exhaustgas device is likely to be poisoned. Therefore, the (B) ZnDTP ispreferably used in an amount not greater than 1.5 parts by weight.

The (C) base oil for engine oil used in the lubricating oil compositionaccording to the present invention is not particularly limited, andknown base oils for engine oil can be employed. At least one kind ofnatural oil or synthetic lubricating oil, or mixtures thereof can beused. Such oils preferably have a viscosity index (VI) of at least 100,more preferably at least 100, and most preferably at least 120.

Examples of such natural oils include animal oils, vegetable oils, oilsobtained from petroleum, paraffin type oils, naphtene type oils,hydrocracked VHVI oils and mixtures thereof. Example of syntheticlubricating oils include olefinic polymers and copolymers such aspolybutylene, polypropylene, propylene-isobutylene copolymers,polybutylene chloride, poly(1-hexene), poly(1-octene), poly1-decene),etc., polyphenyls such as dodecylbenzene, tetradecylbenzene, biphenyl,terphenyl, alkylphenyl, etc., alkyl diphenyl ethers, diphenylalkylsulfate and derivatives thereof, and hydrocarbon oils such asanalogs and homologs, and halogen-substituted hydrocarbons. Examplesfurther include oils obtained by polymerizing ethylene oxide orpropylene oxide, alkyl and aryl ethers of polyoxyalkylene polymersthereof, or mono- or polyvalent carboxylic acid esters or diestersthereof. Diesters obtained from phthalic acid, succinic acid,alkylsuccinic acid and dimers of alkylsuccinic acid, sebacic acid,adipic acid and linolic acid and various alcohols, and polyol estersprepared from polyhydric alcohols, can also be employed. Other :examplesinclude silicic acid type oils such as polyalkylsiloxane oils,polyarylsiloxane oils, polyalkoxysiloxane oils and silicate oils such aspolyaryloxysiloxane oils and silicate oils and liquid esters ofphosphorus-containing acids such as TCP, TOP, diethylesters ofdecylsulfonic acid, etc. Preferred among them are hydrocracked VHVI oiland synthetic oils of polybutene. From the aspect of long drain,hydrocracked VHVI oils having high oxidation stability, mixtures ofhydrocracked VHVI oil and poly-alpha-olefin and/or polyol esters andmixtures of poly-alpha-olefin and polyol esters are particularlypreferred.

Further, the engine oil composition according to the first embodiment ofthe present invention is aimed at improving the MoDTC residual propertyat the time of oil degradation by combining (A) MoDTC and (B) ZnDTPcontaining at least 50 percent by weight of the primary alkyl grouphaving 8 to 14 carbon atoms. When a higher MoDTC residual property isdesired, however, an amine type or phenol type anti-oxidant, metaldetergent, ashless dispersant, etc., are preferably used in combination.

In the antioxidants, examples of the amine type antioxidants includealkylated diphenylamine, phenyl-alpha-naphtylamine,alkylated-alpha-naphtylamine, etc, and examples of the phenol typeantioxidants include 2,6-di-t-butylphenol,4,4-methylene-bis-(2,6-ditertiarybutylphenol), etc. These antioxidantsare generally used in a proportion of 0.05 to 2.0 percent by weight.

Examples of the metallic detergents include phanates, sulfonates,phosphorares, salicylates, etc., of barium (Ba), calcium (Ca) andmagnesium (Mg), as well as perbasic detergents. These detergents aregenerally used in a proportion of 0.1 to 10 percent by weight.

Examples of the ashless detergent/dispersants include benzylamine, boronderivatives of benzylamine, alkenylsucciniimide, boron derivatives ofalkenylsucciniimide, and so forth. These detergent/dispersants aregenerally used in a proportion of 0.5 to 15 percent by weight.

If it is desired that the MoDTC remain, the conjoint use of thehydrocracked VHVI oil is preferred.

Other known extreme-pressure agents, friction mitigators, wear-proofingagents, viscosity index improving agents, rust-proofing agents,fluidization point lowering agents, defoamants, corrosion inhibitors,etc., such as the wear mitigators, e.g., higher aliphatic acids, higheralcohols, amines, esters, etc, and the extreme-pressure agents, e.g.,sulfur type, chlorine type, phosporus type, organometallic type, etc.,may be used in combination in ordinary amounts of use, whenever desired,within the range of the object of the present invention.

Next, in the (D) polyglycerin half esters represented by the generalformula (3) that are used in the engine oil composition according to thesecond invention of the present invention, each of R⁵ to R⁸ represents ahydrogen atom and/or an acyl group having 8 to 20 carbon atoms, but R⁵to R⁸ are never simultaneously the hydrogen atom, nor are theysimultaneously the acyl group. When n is at least 2, n•R⁸ 's exist andin this case, each of such R⁸ 's may be the hydrogen atom and/or theacyl group and may be the same or different. In this specification, apolyhydric alcohol in which part of the hydroxyl groups in said alcoholare esterified will be called a "half ester". The residue of the acylgroup (that is, the residue obtained by removing the carbonyl group fromthe acyl group) may contain a saturated or unsaturated bond(s), and maybe of a straight chain type, a branched chain type, a ring-like type orcombinations thereof.

Examples of such acyl groups include straight chain saturated acylgroups such as a lauryl group, myristyl group, palmityl group, stearylgroup, etc., branched chain saturated acyl groups such as a 2-ethylhexylgroup, isononyl group, isotridecyl group, isostearyl group, etc.,mono-saturated acyl groups such as a linderenyl group (4-dodecenylgroup), tsuzuyl group (4-tetradecenyl group), physetoleyl group(5-tetradecenyl group), myristoleyl group (9-tetradecenyl group),zoomaryl group (9-hexadecenyl group) petroselyl group (6-octadecenylgroup), oleyl group, eleidyl group, gadoleyl group (9-icocenyl group),gondoyl group, etc., poly-unsaturated acyl groups such as a linoleylgroup (9, 12-octadecadienyl group), linoelaidyl group, linolenyl group(9, 12, 15-octadecatrienyl group), eleostearyl group (9, 12,13-octadecatrienyl group), moroctyl group, parinaryl group (9, 11, 13,15-octadecatetraenyl group), arachidonyl group (5, 8, 11,14-icosatetraenyl group), etc., acetyleneacyl groups such as a stearolylgroup (9-octadecynyl group), isanyl group, xymenyl group, etc., cyclicacyl groups such as a hydrocarpyl group, chaulmoogryl group, sterculylgroup, etc., and branched chain acyl groups such as a tuberculostearylgroup.

In the (D) polyglycerin half esters used for the engine oil compositionaccording to the second invention of the present invention, the number(Y) of the acyl groups in the polyglycerin half esters (I), (II) or(III) is within the range of 1≦Y≦ (n+5)/2 and preferably, within therange of 1≦Y≦(n+3)/2. Here, n corresponds to n in the general formula(3). When two or more kinds of half esters are used in combination asthe polyglycerin half esters (I), (II) or (III), Y represents the meannumber of the acyl groups in these two or more kinds of polyglycerinhalf esters. The polyglycerin half esters having Y falling within therange described above are most preferred because the proportion of thehydroxyl groups and the acyl groups exhibits the extreme-pressureproperty. Therefore, where this extreme-pressure property isparticularly required, it is advisable to use an engine oil compositioncontaining the polyglycerin half esters (I), (II) or (III) as theessential components.

Further, in the (D) polyglycerin half esters used for the engine oilcomposition according to the second embodiment of the present invention,the proportion of the lauryl groups and/or the oleyl groups in the totalacyl groups is at least 25% in the polyglycerin half esters (II) or(III). In connection with the acyl groups in the polyglycerin halfesters, the melting point becomes lower as the degree of unsaturationincreases but stability drops, and though the lubrication propertybecomes better with a greater number of carbon atoms, the crystalprecipitates at a low temperature. For these reason, the lauryl groupand the oleyl group are preferred. Where a higher extreme-pressure isrequired than in the case described above, it is preferred to use anengine oil composition comprising the polyglycerin half esters (II) or(III) as the essential constituent components.

In the (D) polyglycerin half esters used for the engine oil compositionaccording to the second invention of the present invention, the acylgroups of the polyglycerin half esters (III) are all oleyl groups and/orlauryl groups. When the polyglycerin half esters are used as theextreme-pressure agent, the oleyl group or the lauryl group is mostpreferred for the reasons described above. Accordingly, when a greaterextreme-pressure polarity is required over the case described above, itis preferred to use an engine oil composition containing thepolyglycerin half eaters (III) as the essential constituent components.

In the (D) polyglycerin half esters used for the engine oil compositionaccording to the second embodiment of the present invention, the amountadded of the polyglycerin half esters (I), (II) and (III) is from 0.1 to5 parts by weight based on 100 parts by weight of the base oil forengine oil as the Component (C). Further, it is possible to use, incombination, at least two kinds of those polyglycerin half esters (I),(II) or (III) whose R⁵ to R⁸ and whose n are different. As to the amountof use in this case, the total amount of the plurality of polyglycerinhalf esters (I), (II) or (III) used must be within the range describedabove.

It has been clarified that these (D) polyglycerin half esters haveexcellent extreme-pressure properties and when they are blended with (A)MoDTC and (B) ZnDTP in a predetermined molar ratio, they exhibit asurprisingly high lubrication property. Concrete examples includediglycerin monolaurate, diglycerin dilaurate, diglycerin trilaurate,diglycerin monooleate, diglycerin dioleate, diglycerin trioleate,diglycerin monolauryl monooleate, diglycerin monolauryl dioleate,diglycerin dilauryl monooleate, tetraglycerin monooleate, tetraglycerinmonolaurate, tetraglycerin monooleyl. monostearate, tetraglycerinmonolauryl monostearate, hexaglycerin monooleate, hexaglycerinmonolaurate, hexaglycerin pentaoleate, hexaglycerin dioleyl distearate,hexaglycerin dioleyl pentastearate, hexaglycerin dilauryl pentastearate,decaglycerin monooleate, decaglycerin monolaurate, decaglycerinpentaolely pentastearate, decaglycerin pentalauryl pentastearate, and soforth. Preferred among them are diglycerin monooleate, diglycerindioleate, diglycerin tetraoleate, tetraglycerin monooleate,tetraglycerin monolaurate, hexaglycerin monooleate, hexaglycerinmonolaurate, hexaglycerin pentaoleate, decaglycerin monooleate,decaglycerin monolaurate, etc.

In an anther aspect of the engine oil composition according to thesecond embodiment of the present invention, the amounts added of the (A)MoDTC, (B) ZnDTP and (D) polyglycerin half esters polyglycerin halfester, polyglycerin half esters (I) or polyglycerin half esters (II)!based on parts by weight of (C) base oil for engine oil are as follows:

    ______________________________________                                        (A) MoDTC          0.03 to 1 part by weight                                   (B) ZnDTP          0.01 to 2 parts by weight                                  (D) polyglycerin half esters                                                                     0.1 to 5 parts by weight                                   ______________________________________                                    

If the amount of each component added is too low, no effects appear andif too great, no effects exceeding a predetermined level appear and onthe contrary, the lubrication property might be adversely affected. Toobtain excellent lubrication properties, therefore, these amounts mustbe essentially satisfied.

Further, when either of the polyglycerin half esters (II) or (III) isused as the polyglycerin half ester in the engine oil compositionaccording to the second invention of the present invention, the amountsadded of (A) MoDTC, (B) ZnDTP and (D) polyglycerin half ester (II) or(III) are as follows:

    ______________________________________                                        (A) MoDTC          0.03 to 1 part by weight                                   (B) ZnDTP          0.01 to 2 parts by weight                                  (D) polyglycerin half ester                                                                      0.1 to 5 parts by weight                                   ______________________________________                                    

total amount of (A)+(B)+(C)=1 to 7 parts by weight;

proportion of (A):(B):(C)=0.1 to 1.5:1:1 to 10.

Blending of these components is preferably made so as to satisfy therelations described above. Higher lubrication performance can beobtained by using them within this range because these additives for thelubricating oil provide a preferable interaction within this range.Accordingly, when the extreme-pressure property is particularlyrequired, this engine oil composition is preferably used.

Further, various known extreme-pressure agents, friction mitigators,wear-proofing agents, etc., such as the friction mitigators typified byhigher fatty acids, higher alcohols, amines, esters, etc., and theextreme-pressure agents typified by sulfur types, chlorine types,phosphorus types, organometallic types, etc., may be used in combinationin ordinary amounts of use within the range of the object of the presentinvention.

Various known additives such as antioxidants typified by phenols andamines, detergents typified by neutral or high basic alkaline earthmetal sulfonates, phenates, carboxylates, etc., dispersants such assucciniimide, benzylamines, etc., viscosity index improving agents suchas high molecular weight polymethacrylates, polyisobutylene,polystyrene, ethylene-propylene copolymers, styrene-isobutylenecopolymers, etc., defoamants such as esters and silicones, and otherrust preventives, fluidization point lowering agents, etc., may besuitably added in ordinary amounts of use within the object of thepresent invention, if necessary.

EXAMPLES

Hereinafter, the present invention will be explained in further detailwith reference to Examples thereof, but the invention is notparticularly limited thereto.

Example A

The engine oil composition according to the first invention of thepresent invention was prepared by using each of the following Samples 1to 17 in the blend proportions described in Table 1, and was subjectedto various tests.

Sample 1: Compound represented by the following formula (A) MoDTC!:##STR3## (wherein R is a 2-ethylhexyl group, and S/O=2.2) Sample 2:Compound represented by the following formula (A) MoDTC!: ##STR4##(wherein R is an isotridecyl group, and S/O=1.5) Sample 3: Compoundrepresented by the following formula (A) MoDTC!: ##STR5## (wherein R isan isotridecyl group and 2-ethylhexyl group, and S/O=2.2 in a molarratio)

Sample 4: Compound represented by the following formula (B) ZnDTP!:

    Zn (RO).sub.2 PS.sub.2 !.sub.2 •aZnO

(wherein R is a primary 2-ethylhexyl group, and a weight ratio of aneutral salt (a=0):basic salt (a=1/3)=55:45)

Sample 5: Compound represented by the following formula (B) ZnDTP!:

    Zn (RO).sub.2 PS.sub.2 !.sub.2 •aZnO

(wherein R is a primary octyl group, and a weight ratio of a neutralsalt:basic salt=68:32)

Sample 6: Compound represented by the following formula (B) ZnDTP!:

    Zn (RO).sub.2 PS.sub.2 !.sub.2 •aZnO

(wherein R is a primary dodecyl group, and a weight ratio of neutralsalt:basic salt=62: 38)

Sample 7: Compound represented by the following formula (B) ZnDTP!:

    Zn (RO).sub.2 PS.sub.2 !.sub.2 •aZnO

(wherein R is a primary tridecyl group, and a weight ratio of neutralsalt:basic salt=85:15)

Sample 8: Compound represented by the following formula (B) ZnDTP!:

    Zn (RO).sub.2 PS.sub.2 !.sub.2 •aZnO

(wherein R is a primary tetradecyl group, and a weight ratio of neutralsalt:basic salt=86:14)

Sample 9: Compound represented by the following formula (B) ZnDTP!:

    Zn (RO).sub.2 PS.sub.2 !.sub.2 •aZnO

(wherein R is a primary hexyl group, and a weight ratio of neutralsalt:basic salt=52:48)

Sample 10: Compound represented by the following formula (B) ZnDTP!:

    Zn (RO).sub.2 PS.sub.2 !.sub.2 •aZnO

(wherein R is a secondary propyl group or n-hexyl group, and a weightratio of neutral salt:basic salt=97:3)

Sample 11: Compound represented by the following formula (B) ZnDTP!:

    Zn (RO).sub.2 PS.sub.2 !.sub.2 •aZnO

(wherein R is a secondary hexyl group, and a weight ratio of neutralsalt:basic salt=97:3)

Sample 12: Phenyl-alpha-naphthylamine

Sample 13: Boric acid derivative of alkenylsucciniimide

Sample 14: (C) base oil for engine oil!

    100 neutral oil (19.9 cSt at 100° C., VI=105)

Sample 15: (C) base oil for engine oil!

    Hydrocracked VHVI oil (18.6 cSt at 100° C., VI=105)

Sample 16: Compound represented by the following formula (MoDTC):##STR6## (wherein R is an isotridecyl group or 2-ethylhexyl group, andX═O) Sample 17: Compound represented by the following formula (MoDTC):##STR7## (wherein R is an isotridecyl group or 2-ethylhexyl group, andX═S)

                                      TABLE 1                                     __________________________________________________________________________    Blending ratio of the engine oil compositions                                 (amount added based on 100 parts by weight of base oil for engine             oil)                                                                                   (A) MoDTC   (B) ZnDTP   (C) Base Oil for                                      Sample                                                                            Amount Added                                                                          Sample                                                                            Amount Added                                                                          Engine Oil                                            No. Parts by Weight                                                                       No. Parts by Weight                                                                       Sample No.                                   __________________________________________________________________________    Example 1                                                                              1   0.4     4   0.94    15                                           Example 2                                                                              1   0.4     5   0.94    15                                           Example 3                                                                              1   0.4     6   0.94    15                                           Example 4                                                                              1   0.4     7   0.94    15                                           Example 5                                                                              1   0.4     8   0.94    15                                           Example 6                                                                              2   0.4     4   0.94    15                                           Example 7                                                                              2   0.4     5   0.94    15                                           Example 8                                                                              2   0.4     6   0.94    15                                           Example 9                                                                              2   0.4     7   0.94    15                                           Example 10                                                                             2   0.4     8   0.94    15                                           Example 11                                                                             3   0.4     4   0.94    15                                           Example 12                                                                             3   0.4     5   0.94    15                                           Example 13                                                                             3   0.4     6   0.94    15                                           Example 14                                                                             3   0.4     7   0.94    15                                           Example 15                                                                             3   0.4     8   0.94    15                                           Example 16                                                                             1   0.1     4   0.94    15                                           Example 17                                                                             1   0.55    4   0.94    15                                           Example 18                                                                             1   0.7     4   0.94    15                                           Example 19                                                                             1   0.4     4   0.6     15                                           Example 20                                                                             1   0.4     4   1.1     15                                           Example 21                                                                             1   0.4     4   1.3     15                                           Example 22                                                                             1   0.4     4   0.94    16                                           Example 23                                                                             2   0.1     4   0.94    15                                           Example 24                                                                             2   0.7     4   0.94    15                                           Example 25                                                                             2   0.4     4   0.6     15                                           Example 26                                                                             2   0.4     4   1.1     15                                           Example 27                                                                             2   0.4     4   1.3     15                                           Example 28                                                                             2   0.4     4   0.94    16                                           Example 29                                                                             3   0.2     4   0.94    15                                           Example 30                                                                             3   0.55    4   0.94    15                                           Example 31                                                                             3   0.8     4   0.94    15                                           Example 32                                                                             3   0.4     4   0.6     15                                           Example 33                                                                             3   0.4     4   1.2     15                                           Example 34                                                                             3   0.4     4   0.94    16                                           Example 35                                                                             1   0.4     4   0.75    15                                                                10  0.19                                                 Example 36                                                                             1   0.4     4   0.56    15                                                                10  0.38                                                 Example 37                                                                             1   0.4     4   0.75    15                                                                11  0.19                                                 Example 38                                                                             1   0.4     4   0.56    15                                                                11  0.38                                                 Example 39                                                                             1   0.05    4   0.66    15                                                                5   0.28                                                 Example 40                                                                             1   0.4     7   0.75    15                                                                11  0.19                                                 Example 41                                                                             1   0.05    4   0.94    15                                           Example 42                                                                             1   0.9     4   0.94    15                                           Example 43                                                                             1   0.4     4   0.1     15                                           Example 44                                                                             1   0.4     4   1.9     15                                           Example 45                                                                             1   0.2     4   0.94    15                                                    3   0.2                                                              Example 46                                                                             1   0.2     4   0.75    15                                                    3   0.2     10  0.19                                                 Comp. Example 1                                                                        1   0.4     10  0.94    15                                           Comp. Example 2                                                                        1   0.4     11  0.94    15                                           Comp. Example 3                                                                        1   0.4     4   0.28    15                                                                10  0.66                                                 Comp. Example 4                                                                        1   0.4     4   0.56    15                                                                11  0.38                                                 Comp. Example 5                                                                        1   0.4                 15                                           Comp. Example 6      4   0.94    15                                           Comp. Example 7                                                                        1   0.01    4   0.94    15                                           Comp. Example 8                                                                        1   0.4     4   0.005   15                                           Comp. Example 9                                                                        1   2.3     4   0.94    15                                           Comp. Example 10                                                                       1   0.4     4   3.0     15                                           Comp. Example 11                                                                       1   0.4     9   0.94    15                                           __________________________________________________________________________

An engine oil oxidation stability test (ISOT test) was carried out bythe following method for each of the engine oil compositions obtainedabove, and measurement of the amount of sludge, measurement of theresidual MoDTC amount by high speed liquid chromatography andmeasurement of the coefficient of friction by an SRV tester were carriedout for the oil after the test. The results are summarized in Table 2.

<Engine Oil Oxidation Stability Test>

The engine oil oxidation stability test was conducted in accordance withJIS K2514 under the following conditions:

    ______________________________________                                        Condition:                                                                    ______________________________________                                        temperature         165.5° C.                                          speed               1,300 rpm/min                                             test time           48 hours                                                  ______________________________________                                    

<Test Measuring Coefficient of Friction>

The test measuring the coefficient of friction was conducted by using anSRV tester under the following conditions:

    __________________________________________________________________________    Condition:                                                                    __________________________________________________________________________    Line contact:                                                                           The test was conducted under a cylinder-on-plate line contact                 condition. An upper cylinder (φ15 × 22 mm) was                      perpendicularly                                                               set to a plate (φ24 × 7.85 mm) in a sliding direction               and was                                                                       allowed to reciprocate so as to measure the coefficient of                    friction. The material of said cylinder and plate was SUJ-2.        load:     200N                                                                temperature:                                                                            80° C.                                                       measurement time:                                                                       15 minutes                                                          amplitude:                                                                              1 mm                                                                cycle:    50 Hz                                                               __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    Lubricating test results of the engine oil compositions                                Coefficient  Residual MoDTC                                                   of Friction  (Mo Content of New                                               New Oil                                                                             Degraded Oil                                                                         Oil as 100%)                                                                            Amount of Sludge                              __________________________________________________________________________    Example 1                                                                              0.065 0.045  67        not greater than 0.08 g                       Example 2                                                                              0.065 0.05   65        not greater than 0.08 g                       Example 3                                                                              0.065 0.055  64        not greater than 0.08 g                       Example 4                                                                              0.06  0.055  65        not greater than 0.08 g                       Example 5                                                                              0.065 0.055  63        not greater than 0.08 g                       Example 6                                                                              0.06  0.05   64        not greater than 0.08 g                       Example 7                                                                              0.06  0.05   62        not greater than 0.08 g                       Example 8                                                                              0.06  0.05   62        not greater than 0.08 g                       Example 9                                                                              0.065 0.06   63        not greater than 0.08 g                       Example 10                                                                             0.06  0.055  61        not greater than 0.08 g                       Example 11                                                                             0.06  0.04   70        not greater than 0.08 g                       Example 12                                                                             0.065 0.05   68        not greater than 0.08 g                       Example 13                                                                             0.065 0.055  67        not greater than 0.08 g                       Example 14                                                                             0.06  0.055  69        not greater than 0.08 g                       Example 15                                                                             0.065 0.055  67        not greater than 0.08 g                       Example 16                                                                             0.075 0.075  57        not greater than 0.08 g                       Example 17                                                                             0.065 0.045  67        not greater than 0.08 g                       Example 18                                                                             0.065 0.045  67        0.1 g                                         Example 19                                                                             0.065 0.055  55        not greater than 0.08 g                       Example 20                                                                             0.065 0.045  66        not greater than 0.08 g                       Example 21                                                                             0.065 0.05   67        not greater than 0.08 g                       Example 22                                                                             0.065 0.04   71        not greater than 0.08 g                       Example 23                                                                             0.075 0.075  57        not greater than 0.08 g                       Example 24                                                                             0.06  0.05   64        0.1 g                                         Example 25                                                                             0.065 0.06   47        not greater than 0.08 g                       Example 26                                                                             0.06  0.055  64        not greater than 0.08 g                       Example 27                                                                             0.065 0.055  64        0.1 g                                         Example 28                                                                             0.06  0.045  69        not greater than 0.08 g                       Example 29                                                                             0.07  0.07   65        not greater than 0.08 g                       Example 30                                                                             0.06  0.04   70        not greater than 0.08 g                       Example 31                                                                             0.06  0.04   70        0.1 g                                         Example 32                                                                             0.06  0.05   57        not greater than 0.08 g                       Example 33                                                                             0.06  0.045  70        0.1 g                                         Example 34                                                                             0.055 0.035  73        not greater than 0.08 g                       Example 35                                                                             0.063 0.06   65        0.15 g                                        Example 36                                                                             0.065 0.055  60        0.18 g                                        Example 37                                                                             0.065 0.05   65        0.15 g                                        Example 38                                                                             0.065 0.05   57        0.18 g                                        Example 39                                                                             0.065 0.045  66        0.15 g                                        Example 40                                                                             0.06  0.055  64        not greater than 0.08 g                       Example 41                                                                             0.075 0.08   50        not greater than 0.08 g                       Example 42                                                                             0.06  0.045  67        0.1 g                                         Example 43                                                                             0.06  0.07   42        not greater than 0.08 g                       Example 44                                                                             0.065 0.055  65        0.1 g                                         Example 45                                                                             0.065 0.04   67        not greater than 0.08 g                       Example 46                                                                             0.065 0.05   65        0.15 g                                        Comp. Example 1                                                                        0.06  0.09   10        0.3 g                                         Comp. Example 2                                                                        0.06  0.09   9         0.3 g                                         Comp. Example 3                                                                        0.063 0.135  0         0.25 g                                        Comp. Example 4                                                                        0.06  0.135  0         0.2 g                                         Comp. Example 5                                                                        0.055 0.133  29        not greater than 0.08 g                       Comp. Example 6                                                                        0.13  0.135  0         not greater than 0.08 g                       Comp. Example 7                                                                        0.1   0.135  0         not greater than 0.08 g                       Comp. Example 8                                                                        0.06  0.135  32        not greater than 0.08 g                       Comp. Example 9                                                                        0.065 0.045  67        0.3 g                                         Comp. Example 10                                                                       0.065 0.115  67        0.3 g                                         Comp. Example 11                                                                       0.065 0.09   14        0.1 g                                         __________________________________________________________________________

An antioxidant (Sample 13) and detergent (Sample 14) were added inamounts of 2.0 parts by weight, respectively, to 100 parts by weight ofthe base oil for the engine oil compositions similar to those ofExamples 1, 6 and 11 (Examples 1', 6' and 11'), and similar tests wereconducted for each of these engine oil compositions. The results aresummarized in Table 3.

                                      TABLE 3                                     __________________________________________________________________________    Amounts added to 100 parts by weight and test results                         Coefficient        Residual MoDTC (%)                                         of Friction        (Mo amount of new                                          New Oil     Degraded Oil                                                                         oil as 100%)                                                                            Sludge Amount                                    __________________________________________________________________________    Example 1'                                                                          0.065 0.04   70        not greater than 0.08 g                          Example 6'                                                                          0.065 0.045  67        not greater than 0.08 g                          Example 11'                                                                         0.065 0.050  73        not greater than 0.08 g                          __________________________________________________________________________

<Copper Plate Corrosion Test and Test Measuring Coefficient of Friction>

To conduct the copper plate corrosion test, to 100 parts by weight ofSample 14, 0.4 parts by weight of each of Samples 1, 2, 3, 16 and 17,and 0.04 parts by weight of Sample 4 were dissolved, respectively, and acopper plate was immersed and heated at 100° C. for 3 hours to test thecorrosion property to the copper plate (in accordance with ASTM D 130).

The test measuring the coefficient of friction was carried out in thesame way as above. The results are summarized in Table 4.

                  TABLE 4                                                         ______________________________________                                                          Degree of                                                                     Copper Plate                                                                              Coefficient                                               Sample  Discoloration                                                                             of Friction                                     ______________________________________                                        Example 47  1         1a          0.06                                        Example 48  2         1a          0.06                                        Example 49  3         1a          0.06                                        Comp. Example 12                                                                          16        1a          0.10                                        Comp. Example 13                                                                          17        3b          0.06                                        ______________________________________                                    

Example B

The engine oil composition according to the second invention of thepresent invention was prepared by using the same sample as the one usedfor Example A with the exception of the Samples described below, in theblending proportion summarized in Table 6, and various tests wereconducted.

Sample 18: Compound represented by the following formula (B) ZnDTP!:

    Zn (RO).sub.2 PS.sub.2 !.sub.2 •aZnO

(wherein R is a primary dodecyl group, and a weight ratio of neutralsalt:basic salt=62:38)

Sample 19: Compound represented by the following formula (B) ZnDTF!:

    Zn (RO).sub.2 PS.sub.2 !.sub.2 •aZnO

(wherein R is a secondary hexyl group and isopropyl group its ratio is1/1, and a weight ratio of neutral salt:basic salt=60:40)

Sample 20 to 35: (D) polyglycerin half esters

                  TABLE 5                                                         ______________________________________                                                          In general formula (3)                                                        R.sup.5 to R.sup.8                                                                      Number                                                                              Number                                      Sample                                                                              Sample Name     x     of H  of acyl group                               ______________________________________                                        20    Diglycerin monooleate                                                                         1     3     oleyl group                                                                           1                                   21    Hexaglycerin monooleate                                                                       5     7     oleyl group                                                                           1                                   22    Hexaglycerin trioleate                                                                        5     5     oleyl group                                                                           3                                   23    Hexaglycerin dioleate                                                                         5     6     oleyl group                                                                           2                                   24    Hexaglycerin monolaurate                                                                      5     7     lauryl group                                                                          1                                   25    Triglycerin dioleate                                                                          2     3     oleyl group                                                                           2                                   26    Diglycerin dioleate                                                                           1     2     oleyl group                                                                           2                                   27    Decaglycerin monooleate                                                                       9     11    oleyl group                                                                           1                                   28    Hexaglycerin pentaoleate                                                                      5     3     oleyl group                                                                           5                                   29    Decaglycerin monolaurate                                                                      9     11    lauryl group                                                                          1                                   30    Tetraglycerin monooleate                                                                      3     5     oleyl group                                                                           1                                   31    Diglycerin tetraoleate                                                                        1     0     oleyl group                                                                           4                                   32    Diglycerin monooleate                                                                         0           oleyl group                                                                           1                                   33    Glycerin dioleate                                                                             0           oleyl group                                                                           2                                   34    Sorbitan monooleate                                                                           --          oleyl group                                                                           1                                   35    Sorbitan dioleate                                                                             --          oleyl group                                                                           2                                   ______________________________________                                    

Sample 36: (C) Base oil for engine oil

The base oil for engine oil used was prepared by adding 4 percent byweight of polymethacrylate as a viscosity index improving agent to a 150neutral oil (5.1 cSt at 100° C.).

The amounts of addition of (A), (B) and (D) shown in Tables 6-1 and 6-2represent the amounts (parts by weight) based on 100 parts by weight ofthe base oil for engine oil.

                                      TABLE 6                                     __________________________________________________________________________            (A) MoDTC                                                                             (B) ZnDTP                                                                             (D)                                                               Amount  Amount  Amount                                                                            (A):(B):(D)                                                                         Total                                           Sample                                                                            Added                                                                             Sample                                                                            Added                                                                             Sample                                                                            Added                                                                             Weight Ratio                                                                        Amount                                  __________________________________________________________________________    Example 47                                                                            1   0.4 4   0.9 20  1.0 0.4:1:1.1                                                                           2.3                                     Example 48                                                                            1   0.4 4   0.9 21  1.0 0.4:1:1.1                                                                           2.3                                     Example 49                                                                            1   0.4 4   0.9 22  1.0 0.4:1:1.1                                                                           2.3                                     Example 50                                                                            1   0.4 4   0.9 23  1.0 0.4:1:1.1                                                                           2.3                                     Example 51                                                                            1   0.4 4   0.9 24  1.0 0.4:1:1.1                                                                           2.3                                     Example 52                                                                            1   0.4 4   0.9 25  1.0 0.4:1:1.1                                                                           2.3                                     Example 53                                                                            1   0.4 4   0.9 26  1.0 0.4:1:1.1                                                                           2.3                                     Example 54                                                                            1   0.4 4   0.9 27  1.0 0.4:1:1.1                                                                           2.3                                     Example 55                                                                            1   0.4 4   0.9 28  1.0 0.4:1:1.1                                                                           2.3                                     Example 56                                                                            1   0.4 4   0.9 29  1.0 0.4:1:1.1                                                                           2.3                                     Example 57                                                                            1   0.4 4   0.9 30  1.0 0.4:1:1.1                                                                           2.3                                     Example 58                                                                            2   0.4 4   0.9 26  1.0 0.4:1:1.1                                                                           2.3                                     Example 59                                                                            3   0.4 4   0.9 26  1.0 0.4:1:1.1                                                                           2.3                                     Example 60                                                                            2   0.4 18  0.9 26  1.0 0.4:1:1.1                                                                           2.3                                     Example 61                                                                            3   0.4 19  0.9 26  3.0 0.4:1:3.3                                                                           3.3                                     Example 62                                                                            1   0.1 18  0.9 26  1.0 0.1:1:1.1                                                                           2.0                                     Example 63                                                                            3   0.9 19  0.9 26  1.0 1:1:1.1                                                                             2.8                                     Example 64                                                                            1   0.4 4   1.9 26  2.1 0.2:1:1.1                                                                           4.4                                     Example 65                                                                            1   0.4 4   0.2 26  1.8 2:1:9 2.3                                     Example 66                                                                            1   0.4 4   0.9 25  0.5                                                                       26  0.5 0.4:1:1.1                                                                           2.3                                     Example 67                                                                            2   0.45                                                                              18  0.3 26  0.3 1.5:1:1                                                                             1.05                                    Example 68                                                                            3   0.4 4   0.9 26  4.5 0.4:1:5                                                                             5.8                                     Example 69                                                                            1   0.4 19  0.9 26  1.0 0.4:1:1.1                                                                           2.3                                     Example 70                                                                            1   0.4 19  0.9 27  1.0 0.4:1:1.1                                                                           2.3                                     Example 71                                                                            1   0.4 19  0.9 24  1.0 0.4:1:1.1                                                                           2.3                                     COMP.                                                                              12         4   0.9 26  1.0 --:1:1.1                                                                            1.9                                          13 1   0.4         20  1.0 0.4:--:1.1                                                                          1.2                                          14 1   0.4 4   0.9         0.4:1:--                                                                            1.3                                          15 1   0.005                                                                             4   0.9 21  1.0 0.005:1:1.1                                                                         1.905                                        16 1   0.4 4   0.9 26  0.05                                                                              0.5:1:0.08                                                                          1.35                                         17 1   0.4 4   0.9 31  0.1 0.5:1:0.16                                                                          1.4                                          18 1   0.4 4   0.9 32  1.0 0.4:1:1.1                                                                           2.3                                          19 1   0.4 4   0.9 33  1.0 0.4:1:1.1                                                                           2.3                                          20 1   0.4 19  0.9 34  1.0 0.4:1:1.1                                                                           2.3                                          21 1   0.4 19  0.9 32  0.5                                                                       33  0.5 0.4:1:1.1                                                                           2.3                                          22 1   0.4 19  0.9 34  0.5                                                                       35  0.5 0.4:1:1.1                                                                           2.3                                          23 1   0.4 19  0.9 35  1.0 0.4:1:1.1                                                                           2.3                                          24 1   0.4 4   0.9 31  1.0 0.4:1:1.1                                                                           2.3                                          25 1   0.4 4   0.9 26  8.0 0.4:1:8.9                                                                           9.3                                          26 1   0.1 18  0.9 26  6.0 0.1:1:6.7                                                                           7.0                                     __________________________________________________________________________

The seizure test and the measurements of the coefficient of frictionwere carried out on the engine oil compositions as the products of thepresent invention and as Comparative Examples, each having the blendingratios shown in Tables 6-1 and 6-2. The results are summarized in Table7.

<Seizure Test>

The seizure test was conducted by using a Falex tester in accordancewith ASTM D 3233. The initial oil temperature was 25° C. and aconditioning operation was carried out at 250 lb×5 minutes.

<Measurement of Coefficient of Friction>

The measurement of the coefficient of friction was conducted under thefollowing conditions by using a pendulum tester.

    ______________________________________                                        Conditions:                                                                   ______________________________________                                        oil temperature:       80° C.                                          number measurements:   50 times                                               ______________________________________                                    

The coefficient of friction was a mean value of 50 measurements.

                  TABLE 7                                                         ______________________________________                                        Lubricating test result                                                                   Farex Test:                                                                           Pendulum Test:                                                        Seizure Load                                                                          Coefficient of Friction                                   ______________________________________                                        Example 47    1800 lb   0.092                                                 Example 48    1750 lb   0.093                                                 Example 49    1750 lb   0.091                                                 Example 50    1800 lb   0.092                                                 Example 51    1750 lb   0.093                                                 Example 52    1800 lb   0.091                                                 Example 53    1850 lb   0.091                                                 Example 54    1750 lb   0.093                                                 Example 55    1750 lb   0.093                                                 Example 56    1750 lb   0.093                                                 Example 57    1750 lb   0.093                                                 Example 58    1850 lb   0.090                                                 Example 59    1850 lb   0.091                                                 Example 60    1850 lb   0.090                                                 Example 61    1800 lb   0.092                                                 Example 62    1700 lb   0.095                                                 Example 63    1900 lb   0.089                                                 Example 64    1750 lb   0.092                                                 Example 65    1750 lb   0.092                                                 Example 66    1800 lb   0.092                                                 Example 67    1800 lb   0.093                                                 Example 68    1800 lb   0.092                                                 Example 69    1850 lb   0.092                                                 Example 70    1750 lb   0.093                                                 Example 71    1750 lb   0.093                                                 Comp. Example 12                                                                            1050 lb   0.285                                                 Comp. Example 13                                                                            1350 lb   0.230                                                 Comp. Example 14                                                                            1400 lb   0.230                                                 Comp. Example 15                                                                            1350 lb   0.280                                                 Comp. Example 16                                                                            1050 lb   0.230                                                 Comp. Example 17                                                                            1400 lb   0.240                                                 Comp. Example 18                                                                            1400 lb   0.230                                                 Comp. Example 19                                                                            1450 lb   0.210                                                 Comp. Example 20                                                                            1450 lb   0.200                                                 Comp. Example 21                                                                            1450 lb   0.180                                                 Comp. Example 22                                                                            1450 lb   0.180                                                 Comp. Example 23                                                                            1400 lb   0.210                                                 Comp. Example 24                                                                            1450 lb   0.230                                                 Comp. Example 25                                                                            1450 lb   0.250                                                 Comp. Example 26                                                                            1450 lb   0.230                                                 ______________________________________                                    

Further, oxidation stability tests were conducted for the engine oilcompositions of Examples 47 to 49 and Comparative Examples 12 and 16 bythe following method. The results are summarized in Table 8.

<Oxidation Stability Test>

The oxidation stability test was carried out in accordance with JIS K2514. After each sample oil was degraded by setting the temperature of athermostat to 165.5° C. and rotating a sample stirring rod at 1,300 rpmto stir for 24 hours, the seizure test was carried out for each oilbefore and after the test. Similar tests were also carried out forengine oil compositions obtained by only replacing the base oil for theengine oil by a hydrocracked VHVI oil (18.6 cSt at 100° C.) for Examples47 to 49 and Comparative Examples 12 and 16. These examples are calledExamples 47*, 48*, 49* and Comparative Examples 12*, 16*, respectively.The seizure test was carried out under the conditions described above.

                  TABLE 8                                                         ______________________________________                                        Lubricating test results                                                                  Farex Test (Seizure Load)                                                     Before ISOT Test                                                                        After ISOT Test                                         ______________________________________                                        Example 47    1800 lb     1300 lb                                             Example 48    1750 lb     1300 lb                                             Example 49    1750 lb     1300 lb                                             Example 47*   1800 lb     1500 lb                                             Example 48*   1750 lb     1500 lb                                             Example 49*   1750 lb     1500 lb                                             Comp. Example 12                                                                            1050 lb      550 lb                                             Comp. Example 16                                                                            1050 lb      550 lb                                             Comp. Example 12*                                                                           1050 lb      850 lb                                             Comp. Example 16*                                                                           1050 lb      900 lb                                             ______________________________________                                    

It became obvious from the results described above that when the baseoil for the engine oil was replaced by the hydrocracked VHVI oil,oxidation stability could be improved.

Effects of the Invention

The first embodiment of the present invention provides an engine oilcomposition which provides low friction and low wear when it is a newoil, and even at the time of oil degradation, has a large residual MoDTC(A) and hence, provides low friction and low wear for a long term.

The second embodiment of the present invention provides an engine oilcomposition which provides an excellent coefficient of friction fromboundary lubricating condition to fluid lubricating condition.

We claim:
 1. An engine oil composition obtained by blending components, including as essential components:(A) at least one molybdenum dithiocarbamate represented by the following general formula (1): ##STR8## (wherein each of R¹, R², R³ and R⁴ may be the same or different and each represent an alkyl group having 8 to 16 carbon atoms, X represents a sulfur atom or oxygen atom, and the ratio of the sulfur atoms to oxygen atoms is from 1:3 to 3:1); (B) at least one neutral or basic zinc dithiophosphate expressed by the following general formula (2):

    Zn (RO).sub.2 PS.sub.2 !.sub.2 •aZnO                 (2)

(wherein a is 0 or 1/3 and R represents an alkyl group having 3 to 14 carbon atoms and may be the same or different), wherein the proportion of zinc dithiophosphate containing a primary alkyl group having 8 to 14 carbon atoms, all R's of which may be the same or different, is at least 50 wt. %; (D) at least one polyglycerin half ester represented by the following general formula (3): ##STR9## (wherein n is an integer of 1≦n≦9, R⁵ to R⁸ each represent a hydrogen atom or an acyl group having 8 to 20 carbon atoms with the provision that all of R⁵ to R⁸ are never simultaneously either the hydrogen atom or the acyl group, and individually R⁸ 's may be the same or different when n is 2 or more, and the number of acyl groups is expressed as (Y); and (C) a base oil for engine oil;wherein the proportion of said Component (A) is 0.03 to 1 part by weight, the proportion of said Component (B) is 0.01 to 2 parts by weight, and the proportion of said Component (D) is 0.1 to 5 parts by weight, based on 100 parts by weight of said base oil for engine oil.
 2. The engine oil composition according to claim 1, wherein all R's in the general formula (2) are 2-ethylhexyl groups.
 3. The engine oil composition according to claim 1, wherein the number (Y) of the acyl groups of the polyglycerin half ester expressed by the general formula (3) is within the range of 1≦Y≦(n+5)/2.
 4. The engine oil composition according to claim 3, wherein the proportion of a lauryl group and/or oleyl group in the total acyl groups of the polyglycerin half ester represented by the general formula (3) is at least 25%.
 5. The engine oil composition according to claim 1, wherein the proportion of a lauryl group and/or oleyl group in the total acyl groups of the polyglycerin half ester represented by the general formula (3) is at least 25%.
 6. The engine oil composition according to claim 5, wherein all the acyl groups of the polyglycerin half ester expressed by the general formula (3) are the lauryl group and/or oleyl groups. 